The present invention relates to a method of writing patterns and also a method of forming patterns.
Electron beams are superior to light beams in forming fine patterns, for example, on sample semiconductor devices. Moreover, electron-beam pattern formation creates masks for optical lithography.
FIG. 1 illustrates an electron-beam writing system.
Electron beams emitted by an electron gun 1 are converged by a convergent lens 2 and radiated onto a first image-forming aperture 3. An image on the aperture 3 is projected on a second image-forming aperture 5 via a projection lens 4.
Provided inside the projection lens 4 is a deflector 6 that adjusts the location of the image on the second aperture 5, that has originally been formed on the first aperture 3, thus producing rectangular or triangular beams of required size.
The image formed on the second aperture 5 is scaled down by an objective lens 7 and projected on a target reticle mask 8, for example.
Electron beams radiated onto the reticle mask 8 are deflected to the center of a sub-deflection zone 11 by a high-precision main deflector 9. The beams are subjected to fine positional adjustments in the sub-deflection zone 11 by a high-speed sub-deflector 10.
On the reticle mask 8, the electron beams travel over frames 12 by stage step movements while on each frame 12 by stage continuous movements. These movements are performed alternately.
Discussed below is a proximity effect, or pattern-size errors, observed in the electron-beam writing system.
As illustrated in FIG. 2, electrons 14 incident to a target 13 are scattered therein and generate secondary electrons. A resist 16 formed on the target 13 is exposed to backscattering electrons 15 among the secondary electrons and scattering electrons. This results in background exposure in addition to exposure by the incident electrons. The region of the resist 16 to be exposed is, for example, about 10 μm in radius under a 50 KeV-writing system.
The exposure of the resist 16 to the backscattering electrons 15 depends on pattern density. Moreover, the pattern size after resist development depends on exposure to the incident electrons 14 and also the backscattering electrons 15. Therefore, the pattern size after development varies in accordance with a pattern density.
This pattern-size variation is called a proximity effect. The proximity effect is also caused by unfocused beams or scattering of electrons in a resist.
Discussed further is along-range fogging exposure which also causes pattern-size errors, observed in the electron-beam writing system.
When electrons 14 are incident to the target 13, as illustrated in FIG. 3, some of the electrons 14 and also secondary electrons are emitted from the target 13 and return to the lower surface 18 of the objective lens 17.
The returned electrons are reflected at the lower surface 18 as reflected electrons 19. The resist 16 is further exposed to the reflected electrons 19, which is background exposure.
This phenomenon is called a long-range fogging exposure. This exposure covers the region of several tens of millimeters from a beam-radiated point on the target 13. A large variation in average amount of beams radiated on the target 13 within about several millimeters thus causes a large variation in resist pattern size after development.
Moreover, a pattern-size variation is caused for patterns etched on the target 13 using a resist pattern as a mask, due to unsteady advancement of etching mainly depending on a pattern density. This phenomenon is called a loading effect.
The pattern-size variation due to the loading effect largely depends on a resist pattern density.